The standoff between Thai protesters and government supporters intensified today as hundreds of protesters stormed Bangkok's Suvarnabhumi international airport terminal after scuffling with riot police.

Members of the People's Alliance for Democracy (Pad) – some masked and armed with metal rods - stormed police lines and roamed through the sprawling $4 billion terminal as startled tourists looked on.

The airport was forced to cancel all departing flights as protesters surged into terminal.

Airport manager Serirat Prasutanon said airport authorities had tried to negotiate with the protesters "but to no avail".

"For the safety of passengers we have to stop flights out of the airport temporarily until the situation returns to normal," he said, adding that incoming flights were still operating.

The governor of Samut Prakan province, where the airport is located, has asked the army to help police. The army spokesman could not be reached for comment.

The attack on the airport follows an incident earlier in the day in which Thai protesters opened fire on government supporters as the six-month campaign to unseat the elected administration continued.

The fighting began when government supporters began throwing rocks at a truck carrying members of Pad on its return from Bangkok's old airport where the group had been holding a rally.

Television footage from public broadcaster TPBS showed Pad protesters responding with handguns and slingshots.

The anti-government protesters had already formed a blockade using trucks and cars on the road leading to Don Muang airport, north of the city. Ministers have been running the country from this area since Pad members invaded Government House in August.

The protesters on the truck appeared to be among the so-called Alliance Guards, who have earned a reputation for aggressive behaviour.

They hoisted a portrait of King Bhumibol Adulyadej, Thailand's revered monarch, whose interests they claim to be working for.

The Narenthorn medical centre said four people were admitted after the clash.

Other television footage showed anti-government supporters surrounding a motorcycle taxi driver and putting a knife to his throat. After the driver fled, the protesters hit several motorbikes with steel rods and set fire to another one.

Government supporters, distinguished by their red shirts from the anti-government demonstrators' yellow, were seen fleeing from Pad members.

Deputy Prime Minister Chauwarat Chanweerakul said the government would refrain from using force to end the standoff.

"They need to stop taking over important places like airports. We will try to negotiate with them to end this siege, because it is not acceptable. It is hurting the economy, and it is hurting the country badly."

Army commander General Anupong Paochinda said the army would not resort to violence.

The second day of protests saw demonstrators blocking all but one lane on the expressway to Suvarnabhumi airport, which is used by 13 million tourists every year.

Activists accuse him of being a puppet of his brother-in-law and former Thai prime minister Thaksin Shinawatra, who was accused of corruption and abuse of power and deposed in a military coup in September 2006. Thaksin is due to return from an Asia-Pacific summit in Peru tomorrow.

"It is time to make a clear-cut choice between good and evil, between those who are loyal and traitors," Pad leader Somsak Kosaisuk told 10,000 yellow-shirted supporters waving hand clappers and shouting anti-government slogans.

Members of Pad, which consists of royalist businessmen and academics, see this week's action as the final battle in their attempt to overthrow the elected government.

But analysts believe the latest protests are unlikely to deliver a knock-out blow to the People Power Party (PPP) government. Opinion polls show waning public support for Pad.

Experts think many of the group's powerful backers in Bangkok are getting cold feet about the damage the long-running political crisis is having on the economy. The export-driven country is already struggling to cope with the global financial crisis.

Despite his ties to Thaksin, Somchai's bland, inoffensive personality has proved a hard target for Pad.

Police are eager to avoid a repeat of October 7 when two protesters were killed and hundreds injured in street battles, the worst violence in Bangkok since the army opened fire on pro-democracy demonstrators in 1992.

Bloodshed could trigger another coup only two years after the army removed Thaksin. However, it is unlikely another putsch could heal the fundamental political rift.

Government supporters, distinguished by their red shirts from the anti-government demonstrators' yellow, were seen fleeing from Pad members.

Activists accuse him of being a puppet of his brother-in-law and the former Thai prime minister, Thaksin Shinawatra, who was accused of corruption and abuse of power and deposed in a military coup in September 2006. Thaksin is due to return from an Asia-Pacific summit in Peru tomorrow.

"It is time to make a clear-cut choice between good and evil, between those who are loyal and traitors," the Pad leader Somsak Kosaisuk told 10,000 supporters waving hand clappers and shouting anti-government slogans.

Members of Pad, which consists of royalist businessmen and academics, see this week's action as the final battle in their attempt to overthrow the elected government.

But analysts believe the latest protests are unlikely to deliver a knock-out blow to the People Power party (PPP) government. Opinion polls show waning public support for Pad.

Experts think many of the group's powerful backers in Bangkok are getting cold feet about the damage the long-running political crisis is having on the economy. The export-driven country is already struggling to cope with the global financial crisis.

Despite his ties to Thaksin, Somchai's bland, inoffensive personality has proved a hard target for Pad.

Police are eager to avoid a repeat of October 7 when two protesters were killed and hundreds injured in street battles, the worst violence in Bangkok since the army opened fire on pro-democracy demonstrators in 1992.

Bloodshed could trigger another coup only two years after the army removed Thaksin. However, it is unlikely another putsch could heal the fundamental political rift.